Spectacle glasses serve as a vision aid and are available in diverse forms. Two spectacle glasses are usually connected by a frame and form, together with this, a pair of spectacles. This can be positioned in front of two eyes so that one spectacle glass is arranged in front of each eye.
The function of a spectacle glass is based on the physical properties of an optical lens. The prerequisite is an optically transparent material. A front and a rear side of the spectacle glass are fabricated, especially by grinding, such that they form at least one optical focus by convex and concave shaping. The determination of the focus of a spectacle glass is done according to a vision defect of a person that will ideally be corrected by means of the spectacle glass. In particular, the natural focus of the eye will be corrected by the spectacle glass.
It may be necessary for an age-related vision correction that the spectacle glass have several focal points. The lens of the eye is curved to differing extent, depending on the distance from the object. With aging vision, this elasticity is limited—the lens of the eye becomes hard and thick. This means that the vision defect of the eye for objects at short distance from the eye (near vision range) is different from that for far removed objects (far vision range). In order to see objects clearly at all distances, the focal point of the eye must be corrected by a particular focal point of a spectacle glass depending on the distance of the object from the eye. An obvious and classical solution is to switch to a spectacle glass with a different focal point depending on the distance of the object from the eye. However, it is also possible to design a spectacle glass so that it has several focal points in different zones. Thanks to such a design it is possible to refrain from switching the spectacle glass and instead switch the zone of the spectacle glass by changing the direction of looking.
If a spectacle glass has two different focal points, generally one for the far vision range and one for the near vision range in the lower region of the spectacle glass, this is known as a bifocal spectacle glass. Such glasses are recognizable mostly on account of a visible transition between the two regions. If the focal point differs repeatedly upon changing the angle of looking, especially from top to bottom, one has a multifocal spectacle glass. The focal point can also change continuously.
Various methods are used to manufacture spectacle glasses. In the most simple case, a spectacle glass is made from a flat blank in several work steps. This blank is a level disk of silicate glass or plastic. At first, in this method, the desired geometries of the optical surfaces are produced on both sides of the blank by mechanical working (lathe work, milling, grinding). The surfaces produced after this step have great surface roughness and are optically nontransparent. Therefore, the machining step is followed by a polishing step. The roughness of the surfaces is reduced so much that light is no longer scattered unacceptably and the lens is thus transparent. In addition, the outer contour of the spectacle glass is produced by machining the outer contour of the blank.
Economical standard spectacles today generally have standardized lenses. At first, round circular blanks are made by casting or injection molding. The quality of such blank surfaces is so high that no further machining of the surface is necessary. Only the contour needs to be adapted to a predetermined spectacle frame. Such spectacle glasses generally have both a spherical front side and a spherical back side. By spherical is meant that the surface is a cutout from a sphere, or that all points on a surface lie at a distance of the radius R from a midpoint M of the sphere. Such standardized spectacle glasses, though extremely economical to purchase, usually do not meet the desires for visual acuity in the near and far zone, and across the entire viewing angle from top to bottom and from left to right.
To reduce these quality defects, more expensive spectacle glasses are usually individualized by surveying the eyes and calculating how a spectacle glass has to be constructed in order to optimally correct an existing vision defect. The outcome of the calculation is usually documented by an optician or eye doctor and provided in the form of a prescription to the person who will fabricate the spectacle glass from a blank. Therefore, in the following we shall refer to a blank individualized by prescription as a prescription lens. From the prescription lens, the spectacle glass is then produced by machining the outer contour.
Various methods are considered for making of prescription lenses. For economic reasons, methods are especially adopted that individually finish a blank produced in a large series by cutting methods and polishing. Blanks of plastic are generally created by casting or injection molding, and blanks of silicate glass by lowering, deepening, or drawing down heated glass rounds into shapes. The quality of such blank surfaces, especially the front and back side, is so high that basically no further machining of the surface is needed. However, such a blank has one or more standardized focal points that often do not optimally correct the vision defect of a particular eye. Therefore, the blank or partial regions of it, such as only one side, are individually adapted by finishing to the vision defect of a particular eye. For this, a removal of material is calculated to configure the prescription lens, based on the front-side and back-side surfaces of the blank and a prescription. The parameters of the surface geometry of the blank are essential for this calculation. After this, a finishing can be done on the front and back side or only on one side. A one-sided finishing generally causes lower production costs than a two-sided one, since the finishing labor is lower. If one side is individualized by finishing, we have a so-called prescription surface.
The finishing is done today with modern CNC milling machines, so that even complicated surface configurations can be realized. However, the finishing of a blank is limited in that both the stability of the lens must be preserved, and the radii of curvature and the material thickness of the blank set limits to the changes. Therefore, at least a certain number of different blanks are needed to manufacture spectacle glasses for the largest possible range of vision defects. Spectacle glasses for rare and extremely large vision defects that do not fall in this range have to be manufactured by different production methods, for example, by manufacture from a cube of material.
A manufacture of individualized spectacle glasses from blanks is described, among other things, in EP 0744 646 A1. Various round blanks are used, one side surface of which has already been prepared. These are made by a casting or injection molding process. The front side of the blank in this case is convex and spherical. The prescription surface is calculated on the basis of a prescription provided by an eye doctor and then manufactured by cutting and polishing. It can be spherical, toroidal or multifocal. The desired optical properties of the spectacle glass result ultimately from combinations of the standardized spherical front side and the prescription side and correspond to the given prescription. Finally, the contour of the prescription lens is adapted to the geometry of a selected spectacle frame.
DE 197 01 312 A1 also describes a manufacturing of individualized spectacle glasses. Here, a blank is finished only on the back side. This means that the prescription surface lies on the back side of the blank. The front side, on the contrary, remains unchanged and it is spherical or aspherical. An aspherical surface is characterized in that it can be described by a mathematical function per DIN ISO 10110-12. The radius here varies continuously from a central axis to the outside. The back side of the spectacle glass is calculated by prescription as a multifocal freeform surface and produced by finish machining. Finally, the contour of the prescription lens is adapted to the geometry of a chosen spectacle frame.
DE 101 03 113 A1 likewise describes a rotationally symmetrical and especially a spherical front side of a round blank, but without further configuring it.
However, considerable drawbacks result from a prescription manufacturing according to EP 0744 646 A1, DE 197 01 312 A1 and DE 101 03 113 A1 with certain spectacle frames. Especially modern narrow spectacle glasses occupy only a small part of the blank surface. Thus, a large waste occurs in the manufacturing. Although such spectacle glasses are small, the production costs therefore correspond to those of larger glasses. Furthermore, in order to manufacture two spectacle glasses, a blank has to be placed twice in a CNC machine, which takes time to do, and causes expense.